Configurations to improve the power generation efficiency have been proposed for power supply apparatuses including piezoelectric elements that do not require batteries. For example, Patent Document 1 discloses a power generation apparatus including a power generation unit that generates power in response to application of strain to a piezoelectric element and a rectification unit that rectifies alternating-current (AC) voltage output from the power generation unit to output direct-current (DC) voltage. A configuration is disclosed in which the AC voltage generated by the piezoelectric element is rectified and smoothed with the rectification unit including a diode bridge and a smoothing capacitor and is converted into the DC voltage.
Patent Document 1: International Publication No. 2003/025969.
FIG. 7 and FIG. 8 illustrate a comparative example of the present disclosure. A piezoelectric power generation module 100R includes a piezoelectric element 1, a rectifier circuit 3, a smoothing capacitor 4, a load switch control circuit 5, a load switch 6, an output node N1, and an output node N2. A load 7 is connected between the output node N1 and the output node N2. In such a configuration in which electric charge generated by the piezoelectric element 1 is moved to the load 7 via the rectifier circuit 3, which is a full-wave rectifier circuit, the electric charge is capable of being supplied to a subsequent unit in both an operation to apply pressure to the piezoelectric element 1 and an operation to release the application of the pressure to the piezoelectric element 1. In other words, the electric charge generated by the piezoelectric element 1 is capable of being supplied to a subsequent unit at both positive voltage generated when the piezoelectric element 1 is deformed with respect to reference voltage in a state in which the piezoelectric element 1 is not deformed, and negative voltage generated when the piezoelectric element 1 is moved to the state in which the piezoelectric element 1 is not deformed with respect to reference voltage in a state in which the piezoelectric element 1 is deformed.
However, for example, in a system including a semiconductor integrated circuit as the load 7, when the voltage exceeds a certain threshold voltage in the deformation of the piezoelectric element 1, a high impedance state occurs when power generation voltage is lower than or equal to a minimum voltage at which the load 7 is capable of being normally driven and the voltage output from the piezoelectric element 1 is biased at a certain voltage. Accordingly, when the electric charge generated when the deformation of the piezoelectric element is returned to a normal state is to be continuously supplied to a subsequent unit, the power generation voltage is decreased. In addition, when the capacitance of the piezoelectric element 1 is made higher than that of the smoothing capacitor 4 in order to improve the power generation efficiency, the voltage does not reach the threshold voltage at which the load switch 6 is capable of operating in the worst case. Accordingly, there is a problem in that the piezoelectric power generation module is not capable of being stably driven in both the operation to apply pressure to the piezoelectric element and the operation to release the application of the pressure to the piezoelectric element.